1. Field of the Invention
This invention relates to a radiation image read-out apparatus for exposing a stimulable phosphor sheet, on which a radiation image has been stored, to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, photoelectrically detecting the emitted light, and thereby obtaining an image signal representing the radiation image. This invention particularly relates to a radiation image read-out apparatus wherein a plurality of laser beams are combined, and the combined laser beams are utilized as stimulating rays.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed as desired. The processed image signal is used during the reproduction of a visible image which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced as a hard copy or may be displayed on a cathode ray tube (CRT).
In the radiation image read-out apparatuses for scanning a stimulable phosphor sheet, on which a radiation image has been stored, with stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, and photoelectrically detecting the emitted light, stimulating rays having as high an intensity as possible should be used to allow the speed, with which the stimulable phosphor sheet is scanned with the stimulating rays, to be kept high. For this purpose, as disclosed in, for example, U.S. Pat. No. 4,655,590, laser beams sources and which serve as stimulating rays, may be combined such that beam spots of the laser beams are superposed one upon another on a stimulable phosphor sheet.
When a plurality of laser beams are combined, it is technically difficult for the beam spots of the laser beams to be superposed one upon another on a stimulable phosphor sheet such that their center points completely coincide with one another. In order for the cost of the radiation image read-out apparatus to be kept low, it is advantageous that slight deviation in the center points of the beam spots allowed to occur. In general, no practical problem occurs from slight deviation in the center points of the beam spots.
However, in cases where a high resolution type of stimulable phosphor sheet is used in the radiation image read-out apparatus, problems often occur from slight deviation in the center points of the beam spots. Specifically, when the center points of the beam spots deviate from one another, the effective spot diameter of the stimulating rays becomes large. Therefore, the sharpness of a radiation image read out from the high resolution type of stimulable phosphor sheet becomes low.